Pressure builds to 20 lbs- and wont go any higher and motor runs

most likely a problem reed /poppet valve in the compressor heads

Sounds like your not using enough water, and or your pipes to sprinkler is to to small, causing pump to build up to cut off head. I cant lay my hands on a performance chart for your pump, but 60 psi cut off and 20 on sounds about right. Try running another tap when your sprinkler is running, and see if this keeps your pump running

Does it have an intake air filter? Has the compressor been properly maintained? Oil less? It may have worn out rings and or valves

check air tank drain valve located on bottom of tank, if it is not leaking try adjusting air pres regulator (large black knob close to air hose connection.) the knob when turned in will increase the tank air pressure, if knob wont turn check that lock ring on bottom of knob to make sure it is disengaged. also check for air leaks in all lines and connections

The problem with compressors not building pressure is usually a bad pressure switch or bad regulator. Does the motor shut off at 20psi or does it continue running and not build pressure? If the motor continues to run with no increase in pressure, the you should go over the machine looking for air leaks from your regulator back all the way to the motor. Any air leak while the compressor is running can cause the pressure to "peak" way before the intended shutoff that it is regulated to.

I'd pull the cover off to expose top of cylinder head and have a look at any intake air filter. In parts diagram, It appears there is a small one that would likely clog during the texturing process.

The check valve is not holding the pressure back off the head,therefore allowing the pressure to build up fully.

Check to see if air is leaking past 'pressure relief' valve. It's probably leaking and that's where all the air is going.

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sounds like the belt is slipping have you checked the inlet and discharge valves also could be bad and leaking or stuck or installed wrong

Check the reset button on the motor to see if it tripped. The motor may be overheating. Some motors have an internal breaker and they have to cool off before they restart. Also check the pressure switch. Move the switch back and forth ( on and off ) and listen for a click. If it doesn't click with low to no air pressure then the switch is bad.